US President Donald Trump is set to deliver a speech at South Korea's National Assembly on Wednesday, the first such address by an American head of state in nearly a quarter century.

In his 20-minute address, Trump is expected to talk about his vision for the South Korea-US alliance, North Korea's nuclear standoff, trade relations and possibly his broad policy for the Indo-Pacific region, observers said.

This photo, taken on Nov. 7, 2017, shows U.S. President Donald Trump speaking during a press conference at the presidential office Cheong Wa Dae in Seoul. (Yonhap)

Some 550 people will attend the speech, including South Korean lawmakers, foreign diplomats and those invited by the Washington government.

Since 1960, a total of five US presidents, including Bill Clinton in 1993, have spoken at the assembly, touching on the changing contours of security and geopolitics following the Korean War, the Vietnam War and other consequential historical events.

The common thread running through their speeches was America's firm commitment to its Asian ally's security and free democracy -- anchored on the 1953 mutual defense treaty -- in the face of persistent threats from the communist North.